DVDs (digital versatile discs), which are commonly used optical discs that allow for high-density recording, have a structure in which two substrates having a thickness of 0.6 mm are bonded together with an adhesive. To achieve a high density in DVDs, a laser beam having a short wavelength of 650 nm and an optical system having a high numerical aperture are used, compared with the case of CDs (compact-discs).
However, to record or reproduce, for example, high-definition images for HDTV (high definition television), a further increase in the density is required. Methods for performing higher-density recording for the next generation of DVDs and optical discs to be used with the methods have been studied; and a method for performing high-density recording has been proposed that employs a new optical-disc structure with a blue laser beam having a short wavelength and an optical system having a high numerical aperture, compared with the case of DVDs.
This new optical disc has a structure in which a recording layer is formed on a transparent or opaque substrate composed of a plastic such as polycarbonate; a light transmitting layer having about 100 μm is further stacked on the recording layer; and a recording beam, a reproduction beam, or both of the beams are made incident on the optical disc through the light transmitting layer. In view of productivity, use of an ultraviolet curable composition for a light transmitting layer of such an optical disc has been actively studied.
Such an optical disc used with a blue laser beam needs to keep recording-reproduction characteristics with stability for a long period of time. Accordingly, the light transmitting layer desirably does not adversely affect the recording-reproduction characteristics due to surface deformation or scratching thereof even after being used for a long period of time. For DVDs and CDs, a recording beam or a reproduction beam is made incident on a surface composed of a plastic material such, as polycarbonate. In contrast, for the above-described optical disc, a cured film, of an ultraviolet curable composition serves as an incident surface and the film under a load for a long period of time has a problem of an increase in signal reproduction errors due to deformation or the like thereof, compared with polycarbonate.
To prevent deformation of the light transmitting layer under a load, the light transmitting layer is desirably a rigid cured film. For example, an optical information medium including a light transmitting layer having a dynamic elastic modulus of 1.5 to 3.0 GPa at 25° C. has been disclosed (refer to Patent Literature 1). However, this disc under a load for a long period of time has a problem of an increase in signal reproduction errors.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2003-123316